The present invention relates generally to power supplies, and, more particularly, relates to strobe tube power supplies.
Emergency vehicles such as fire trucks, police vehicles and ambulances rely on sirens and lights to warn civilians and to protect traveling emergency personnel. Strobe lights have higher intensity than ordinary lights and are preferred for emergency vehicle applications. The exigent circumstances of an emergency situation dictate that the sirens and lights on emergency vehicles operate efficiently, reliably and without delay.
Strobe lights require an energy storage capacitor, e.g., a flash capacitor, to produce flash patterns. To charge the flash capacitor to produce a flash pattern, strobe lights typically implement a strobe power supply comprising power switching transistors and other electrical components. Flash capacitors are coupled to a strobe power supply that is installed between one or more flash tubes and a power source. The power supply, flash capacitor and gas-filled strobe tubes cooperate to produce flashes of light. The flash capacitor and strobe tubes are connected directly to each otherxe2x80x94in a parallel circuit arrangement. In common practice, a flash capacitor is charged to a voltage below the ionization voltage of the gas in the tube; the gas remains de-ionized and electrically non-conductive until triggered. To trigger a flash, a relatively high voltage pulse applied to a wire wrapped around the tube initiates ionization of the gas. The charge on the capacitor then completes the ionization, rendering the tube electrically conductive and causing the capacitor to discharge into the ionized gas. The flash capacitor discharging produces the flash. After the capacitor has discharged, the gas de-ionizes provided the charging current from the power supply is turned off for a sufficient time after the discharge. To produce a next flash, the flash capacitor is recharged and the trigger reapplied. Since the capacitor and tube are connected in parallel, a means must be provided to hold off charging current into the flash capacitor for a sufficient time immediately following a flash. Otherwise, charging current will flow into the tube instead of the capacitorxe2x80x94as the tube remains electrically conductive; the charging current will sustain the ionization and the tube will remain electrically conductive until the charging current is turned off for a sufficient time. This diversion of the charging current away from the capacitor and into the tube keeps the capacitor from charging, thereby disabling the flash system. This fault condition is called, xe2x80x9cneoningxe2x80x9d. The term, neoning, derives from the fact that the tube glows dimly, like a neon tube, when provided with a sustained current. The light output from such a neoning strobe tube is inadequate for any practical purpose. Furthermore, just one neoning tube diverts all of the available charging current thereby disabling an entire system of multiple tubes connected to a common strobe power supply. The time needed to de-ionize a tube following a flash is not a well-quantified parameter. Rather, the time varies with tube gas pressure and other ill-quantified phenomena. As a tube ages, the propensity to neon increases due to reduced gas pressure caused by leakage at the tube seals. All too often, a defective (neoning) tube disables an entire system of multiple tubes. A method of automatically isolating and effectively disconnecting a neoning tube is highly desirable because such method would keep a system operating even with one or more defective (neoning) tubes.
Given the considerations of emergency vehicles, what is needed is a power supply for strobe lights that is tolerant of defective strobe tubes and provides the requisite light energy for emergency uses. When the power requirement is for more than 60-watt, it is desirable to have a method of synchronizing the switching cycles of dual power converters operating in transitional mode to maintain 180-degrees of phase displacement between the converters.
In light of the above, it is a general aim of the present invention to provide a reliable strobe power supply that provides requisite light energy for emergency uses without causing EMI problems. A dual flyback power converter operating in transitional mode is disclosed that includes a programmable control circuit configured to operate each of the converter""s power switching transistors in response to circuits that enable a small dead time between the cessation of stored energy in the flyback transformers and turn-on of the associated transistor via synchronization code in the programmable control circuit that periodically delays turn on of one or the other transistor to maintain a 180 degree relationship between the two phases.
The power supply is also capable of detecting a fault (neoning) condition in a system of strobe tubes by measuring flash capacitor voltage subsequent to a flash and identifying a neoning condition as a state in which the flash capacitor voltage fails to increase after 10 mS of flash capacitor charging.
The power supply is also capable of automatically correcting a fault (neoning) condition by incrementing the flash capacitor charge off-time delay to the off-time delay needed to prevent a fault (neoning) condition.
The power supply is also capable of tolerating defective (persistently neoning) strobe tubes that cause an inordinate delay in capacitor charging in a system by first identifying the defective strobe tubes by individually firing each strobe tube in the system, determining an anti-neon off-time delay suitable for the individual strobe tubes, identifying whether any strobe tube is causing an inordinate delay in capacitor charging; then turning off any such identified strobe tubes.
One embodiment is directed to a strobe power supply that includes an input filter, a programmable control circuit coupled to the input filter, a first and second transistor operatively coupled to the programmable control circuit, a first and second transformer, each transformer operatively coupled to one of the first and second transistors, and two circuits configured to sense an energy state, such as a current state or voltage state of each transformer, the circuits are coupled to the programmable control circuit. The programmable control circuit is configured to operate each transistor in response to at least one of the circuits to provide a small, variable dead time between the cessation of stored energy in the transformers and turn-on of the associated transistor via synchronization code in the programmable control circuit, the synchronization code periodically delaying turn-on of one or the other transistor to maintain a 180 degree phase difference between switching cycles of the first and second transistors. The 180 degree relationship reduces ripple current in the input filter. The programmable control circuit can be configured to provide switching cycle signals to the first and second transistors, the switching cycle signals according to a logical function applied to a combination of turn on commands, the logical function allowing only the later command of a measured synchronizing turn on and a normal turn on for the first transistor to be an operative turn on, the synchronizing turn on command enabling synchronization of the turn on of the first transistor with a phase displaced turn on of the second transistor. In one embodiment, the logical function is equivalent to AND-ing of the turn on commands.
In one embodiment, the strobe power supply includes at least two isolating circuits coupled to the programmable control circuit. Each of the isolating circuits can include a voltage divider configured to provide a voltage measurement of a flash capacitor and to provide for a voltage limiting function for a flash lamp.
One embodiment is directed to operating two power converters in two phases with transitional conduction mode for a strobe power supply. The method includes periodically introducing a small dead time to the higher frequency power converter to maintain a constant phase angle displacement between the two phases. In one embodiment of a two-phase power supply, the method includes adjusting the two phases to a displacement of 180 degrees at least once every six power cycles of the combined converters. The method also includes measuring a period of a phase according to a time between each turn on of a transistor in at least one of the power converters and dividing the measured period by two. A final embodiment is directed to a method for synchronizing phases of a dual power converter in a strobe power supply. The method includes measuring the period of a first phase of the dual power converter then dividing the period by two to obtain the half-period, waiting for the half-period of time, issuing a turn on command, and AND-ing the turn on command with a turn on command for the second phase of the dual power converter. The period measurement, dividing by two, and half-period wait followed by application of the synchronizing turn on command can occur every fourth cycle of each phase.
The programmable control circuit can apply a logical function such as AND-ing to a combination of turn on commands, the logical function allowing only the later command of a measured synchronizing turn on and a normal turn on for a first transistor to be an operative turn on, the synchronizing command enabling synchronization of the turn on of the first transistor with a phase displaced turn on of a second transistor in a out of phase power converter. The periodic introduction of dead time can be determined via an external interrupt service routine including a first external interrupt occurring at a cessation of secondary current for a first power converter and a second external interrupt occurring at a cessation of secondary current for a second power converter, the first and second external interrupts identifying the corresponding transistor to turn on. The first and second external interrupts and a flags variable can determine which cycle of the six-cycle synchronization cycle of the two power converters is enabled.
One embodiment is directed to a method for detecting a neoning condition in a strobe power supply. The method includes measuring flash capacitor voltage subsequent to a flash and identifying a neoning state when the flash capacitor voltage fails to increase by a predetermined amount after 10 mS of flash capacitor charging. If neoning is identified, the method includes incrementing an anti-neon off-time delay by a predetermined amount, immediately turning off a charge current for the incremented delay time, after the incremented delay time, turning on the charge current, and after a predetermined amount of on time, rechecking the flash capacitor voltage. If the flash capacitor voltage rises, the method includes applying the incremented delay time to each subsequent flash; and if a predetermined failure delay time is reached, applying a diagnostic sequence to identify and remove defective strobe tubes.
One embodiment is directed to a system for diagnosing and correcting neoning in a strobe tube power supply. The system includes a programmable control circuit configured to operate computer code. The computer code includes an anti-neon off-time delay variable configured to store a value capable of being incremented by a predetermined delay time, an output from the programmable control circuit configured to supply a charge current to one or more flyback converters within the strobe tube power supply, the programmable control circuit configured to turn off the charge current for the time equivalent of the value stored in the off time delay variable, and one or more flash capacitors coupled to the flyback converters. The programmable control circuit can be configured to test one or more voltages of the one or more flash capacitors, the code within the programmable control circuit configured to determine whether any flash capacitor voltage has failed to increase, the failure indicative of a neon condition, the programmable control circuit configured to respond to the failure by increasing the value stored in the off-time delay variable. The two flyback converters can be operated out of phase by 180 degrees, the programmable control circuit being configured to maintain the 180 degree phase difference between the two flyback converters.
One embodiment is directed to a method for tolerating defective (persistently neoning) strobe tubes that cause an inordinate delay in capacitor charging in a system by first identifying the defective strobe tubes by individually firing each strobe tube in the system, determining an anti-neon off-time delay suitable for the individual strobe tubes, identifying whether any strobe tube is causing an inordinate delay in capacitor charging; turning off any such identified strobe tubes; and determining an anti-neon off-time delay suitable for the remaining strobe tubes. The method includes selecting a flash tube from a list of active flash tubes within the system, testing the selected flash tube to determine a delay for the selected flash tube or to turn off the selected flash tube, repeating the testing for each flash tube in the list of active flash tubes, and removing turned off flash tubes from the list of active flash tubes, the list of active flash tubes stored in a programmable control circuit. Prior to selecting the flash tube, an embodiment of the method includes incrementing a system delay time until a voltage for a flash capacitor within the flash strobe power supply system rises, and resetting the system delay time to a start-up value. The testing includes operating the flash tube to determine a required delay for the selected flash tube, if the required delay is over a predetermined limit, turning off the selected flash tube and removing the selected flash tube from the list of active flash tubes within the system, and if the required delay is within the predetermined limit, selecting another flash tube from the list of active flash tubes.
In one embodiment, a programmable control circuit performs the comparing, identifying, turning off and determining of the delay time.
A final embodiment is directed to a method for synchronizing phases of a dual power converter in a flash strobe power supply. The method includes dividing a period of a first portion of the dual power converter and obtaining a predetermined period of time relative to 180 degrees, waiting for the predetermined period of time, issuing a turn on command, and AND-ing the turn on command with a turn on command for the second portion of the dual power converter. The predetermined period of time can be a half period, and the dividing can occur every fourth cycle of each phase.
Other objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.